Victor Perry, of Ngahere School, asks :-

How long does it take for Tb to develop, and do birds have a different disease to mammals?

Sandy Smith, a microbiologist at Otago University's School of Medical Sciences, responded.

Tuberculosis in humans is usually caused by the germ (bacterium) 'Mycobacterium tuberculosis', although closely related species such as 'Mycobacterium bovis' and 'Mycobacterium avium' may occasionally be responsible. These latter two germs have other mammals (e.g. cattle, deer, possums, ferrets) and birds as their normal hosts respectively.

Tuberculosis, or Tb, can thus be an important zoonosis i.e. a disease usually found in lower animals but transmissible to humans following contact with a diseased animal. This is one of the main reasons why we now pasteurise milk from cows, and why all cattle and deer in New Zealand killed for human consumption must be certified free of tuberculosis.

Tuberculosis usually involves the lungs (a respiratory disease) and is acquired by inhalation of 'M.tuberculosis' germs that have been coughed or sneezed into the air by a person with active lung disease. Following this exposure, most people develop inapparent infection although a few progress to active (overt) disease with lung lesions. This may happen rapidly, i.e. a few weeks, or take many years (e.g. 70 years) to develop.

What happens to the inhaled germs and how fast disease develops depends on the ability of the patient to fight the bacterium. Poor housing conditions and malnutrition favour the development of active lung disease. When patients have serious underlying associated disease, e.g. AIDS, Tb may spread rapidly from the lungs to involve other body organs. Treatment of active Tb is not easy and requires a combination of 3 or 4 drugs for at least 6 months.